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The biosynthesis of ethylene from methionine in Saccharomyces cerevisiae

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  • Methionine metabolism, leading to the biosynthesis of ethylene in Saccharomyces cerevisiae does not appear to involve S-adenosylmethionine (SAM) and aminocyclopropanecarboxylic acid (ACC), based on the lack of ethylene synthesis when both SAM and ACC were incubated in the presence and absence of cells. Thus, the mechanism leading to the formation of ethylene from methionine in Saccharomyces cerevisiae is different than that of higher plants. Incubation of cells without methionine included in the reaction mixture formed no appreciable amounts of ethylene, also, the α-keto (KMB) analogue of methionine produced more ethylene than methionine supplementations in both the absence and presence of cells, and the decarboxylated product of KMB (methional) accumulated approximately four times more than this. There was also no appreciable ethylene accumulation when the or-hydroxy (HMB) analogue of methionine was utilized as a precursor. The determining factors in the accumulation of ethylene from methionine and other substrates were the concentration of substrates and glucose, along with the pH of reaction mixture. The presence or absence of light had a definate effect on ethylene production also, showing no signs of accumulation if reaction mixtures were incubated in the dark. In the light, the only medium component needed for ethylene production from substrates was riboflavin. Radiolabeled methionine in the 2,3-4, and methyl positions revealed the accumulation of ethylene products from carbon numbers 3 and 4 of methionine. The treatment of cell and medium extracts with dansyl hydrazine and 2,4-dinitrophenylhydrazine showed the build up of an aldehyde and/or ketone derivative, with the ketone showing the highest accumulations in the culture medium. Inhibition of ethylene synthesis occured when chloramphenicol, adenosine and azaserine were added to methionine supplemented cultures. Thus, the possibility that protein synthesis is occuring while ethylene synthesis is taking place is supported, along with the need for hydroxyl radicals, which are scavenged if adenosine is present. The inhibition of ethylene synthesis from methionine supplemented cultures with the addition of azaserine imply that transamination is also involved in ethylene biosynthesis from methionine in Saccharomyces cerevisiae.
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